1. Field of the Invention
The present invention relates to an image compressing apparatus in the image processing technology.
2. Related Background Art
There are some international standards defined for compression method of moving image, for example, H. 261 for video phone or video conference system by CCITT (International Telegraph and Telephone Consultative Committee) and MPEG for storage media by ISO (International Organization for Standardization). These international standards employ common techniques of interframe predictive coding with motion detection, orthogonal transform, quantization and variable length coding. The summary of these international standards is described for example in "international standards for multi media coding" (Maruzen, June 1991) edited by Hiroshi, YASUDA.
FIG. 13 shows a conventional image compressing apparatus in conformity with the above international standards. The image compressing apparatus in FIG. 13 is composed of a predictive coding unit 10 for producing prediction errors 31 for each block and a prediction error sum 37 for each macroblock; an orthogonal transform unit 11 for effecting an orthogonal transform on each predictive error 31 to produce an orthogonal transform result 32; a quantization unit 12 for quantizing each orthogonal transform result 32 to produce a quantizing result 33; a variable length coding unit 13 for variable-length-coding each quantizing result 33; and a coding control unit 14 for producing a quantization parameter 35. In the following description, video input signals 30 are represented by Aij, the predictive errors 31 by Bij, the orthogonal transform results 32 by Cij, the quantizing results 33 by Dij, and the quantization parameter 35 by Q.
The image compressing apparatus divides an image frame into processing units called macroblocks, and effects the same processing on each macroblock. FIGS. 2a to 2c are drawings to show the constitution of a macroblock. FIG. 2a shows a luminance signal Y, FIG. 2b a chrominance signal C.sub.B, and FIG. 2c a chrominance signal C.sub.R, which indicates that a macroblock is composed of a luminance signal of 16.times.16 pixels and two types of chrominance signals of 8.times.8 pixels. In the drawings, a circle represents a pixel. Also, a macroblock is further divided into blocks, each with 8.times.8 pixels as shown in FIGS. 2a to 2c. Each block is a minimum unit for processing. Therefore, a macroblock is composed of four blocks of luminance signals and two blocks of chrominance signals. The image compressing apparatus compresses an image by consecutively performing the processes of predictive coding, orthogonal transform, quantization and variable length coding in the unit of a macroblock or block.
In the conventional image compressing apparatus as shown in FIG. 18 and described above, the orthogonal transform unit, the quantization unit and the variable length coding unit are activated to operate for all blocks irrespective of the characteristics of predictive error.
LSI Logic Inc. and Graphics Communication Technology Inc. each independently presented an LSI chip set for image compression in conformity with H. 261 in IEEE Transactions on Circuits and Systems for Video Technology (pp. 111-133, June 1992), for example. Their presentations illustrated that the conventional image compressing apparatus could be constructed using the developed LSI chip set.
Also, NIPPON TELEGRAPH AND TELEPHONE presented another image compressing apparatus in conformity with H. 261 using DSP for image compression in the above Transactions (pp. 207-220) and in NTT R&D (No. 8, pp. 1025-1042, 1991). The image compressing apparatus also falls within the field of the conventional image compressing apparatus.
Further, Japanese Patent Application Laying Open (KOKAI) No. 4-196681 describes a quantization circuit which executes no division if a quantizing result or quantization parameter is 1. An image compressing apparatus using this circuit can simplify merely a part of the operation in the quantization unit.
The conventional image compressing apparatus activates the orthogonal transform unit, the quantization unit and the variable length coding unit for all blocks irrespective of the characteristics of predictive error. Accordingly, such circuits were activated to operate even for a block which has predictive errors small enough to give advance knowledge that all quantizing results are to be 0, which caused an increase in dissipation power of the apparatus and an increase in processing time.